The uranium-arsenic crystal UAs2 under pressure behaves like a house with two rooms. In one room, electrons sit still; in the other, they roam freely. Under normal conditions, it's a classic "lattice Kondo" system: heavy, almost immobile electrons on 5f5/2 orbitals are surrounded by a sea of fast conduction electrons. A flat band emerges, like a shelf where electron velocities are nearly zero. This is exactly what photoemission experiments observe.

Squeeze the crystal, and the moving begins. Electrons from the more localized 5f5/2 orbital migrate to the more mobile 5f7/2 orbital. The total number of electrons on uranium doesn't change: it's like rearranging things from a suitcase into a closet without leaving the room. This orbital-selective charge transfer is self-doping: the material changes its electronic properties on its own, without adding impurities. As a result, electrons transition from a "sit-still" regime to a mixed-valence regime with enhanced charge fluctuations. The electronic structure is drastically rearranged.

And here superconductivity appears — a record for uranium materials. It emerges precisely when the Fermi surface consists of two disconnected sheets. Between them there is "nesting"—a situation where one sheet can be superimposed on the other by a simple shift. This promotes electron pairing. But if pressure increases a bit more, the sheets bend and merge into a three-dimensional cylinder, and superconductivity disappears. Such a sensitive mechanism hints that UAs2 works differently from conventional heavy-fermion superconductors: perhaps charge fluctuations, not magnetic ones, play the leading role. The two-room house turns out to be an ideal stage for a quantum dance of electrons without resistance.